Automatic grounding switch

ABSTRACT

Designed for use with electrically grounded power distribution systems, the switch is operable with replaceable propellent cartridges in association with a current limiting fuse. Upon receipt of an electrical signal indicating the presence of an overcurrent fault, a small surge of energy is provided to fire the cartridge and, in turn, propel its plunger into direct contact with a &#39;&#39;&#39;&#39;hot&#39;&#39;&#39;&#39; terminal of the switch, establishing a conducting path to ground. The magnitude of the resulting short circuit currents to ground are limited by the current limiting fuse.

United States Patent [15] 3,660,720 Samm et al. 1 May 2, 1972 [54]AUTOMATIC GROUNDING SWITCH 3,238,321 3/1966 Lawwill et al ..200/61.083,448,341 6/1969 Casey ..3l7/l6 X [72] Inventors: Ralph W. Samm,Greensburg; E. Robert Perry Scottdalei David nods, Green PrimaryExaminer-James D. Trammell Sburg of Attorney-Ostrolenk, Faber, Gerb &Soffen 73 Assi nee: I-T-E lm erial Cor oration Philadel hia, 1 g Pa. p pp 57 ABSTRACT 22] Filed; 0m 0 9 0 Designed for use with electricallygrounded power distribution systems, the switch is operable withreplaceable propellent pp 82,310 cartridges in association with acurrent limiting fuse. Upon receipt of an electrical signal indicatingthe presence of an 52 U.S.Cl. ..3l7/l6 ZOO/61.08 317/33 c Newman fault,a Surge energy is Pm"ided fire Y317/33 the cartridge and, in turn,propel its plunger into direct con- 511 Int. Cl. ..H02h 3/08 with aterminal of Switch esablishing a [58] Field of Search 317/13 R, 16. 18 RD 33 Sc, ducting path to ground. The magnitude of the resulting short317/33 C 200/61 337l40| 220 circuit currents to ground are limited bythe current limiting fuse. [56] References Cited 7 Claims, 10 DrawingFigures UNITED STATES PATENTS 3,296,494 1/1967 Stenger et a]. ..3l7/l8 XZ IE (Z K/ 64 K4 5 T Patented May 2, 1972 2 Sheets-Sheet 1 FUSE IZEZ- 1l 1 i I I $1 5/ Q/ /7 7 [I i/ I i/ I I NVENT/OL; 84/. PH 14/. 5/1/ 4 5FOfiA-WT PERRY Patented May 2, 1972 2 Shuts-Shut 2 .Z'ChZa- BACKGROUNDOF THE INVENTION 1. Field Of The Invention The present invention relatesto electrical power distribution systems, in general, and to anautomatic grounding switch for use in such systems to protect againstthe generation of small overload currents.

2. Description Of The Prior Art As will be appreciated by those skilledin the art, there exists many instances in the distribution ofelectrical power where the equipment employed cannot readily be fused toadequately protect against small overload currents. One such particularinstance arises with underground transmission systems, which areincreasingly playing a larger role in present distribution design. Notonly does a need exist in such instances to so protect the equipmentemployed, but the protective arrangement most desirable is one which isat the same time small, compact and inexpensive to produce. Due to itsrelatively inaccessible emplacement, it also is apparent that the deviceselected should be reliable in its operation and easy to maintainalthough integrated into an incarcerated installation.

Recognition of the need to protect fused electrical equipment againstsmall overloads has been appreciated. One such device in use has beenknown to incorporate a solid state relaying scheme employing a vacuuminterrupter. Also employed in known arrangements are circuit breakerassemblies with their associated relay equipment. Such designs, however,have either high initial cost or limited current ratings, or are, by andlarge, quite bulky and difiicult to maintain.

SUMMARY OF THE INVENTION As will become clear hereinafter, the switch ofthe instant invention is designed for use in an electrically groundedtransmission system and is selected to operate in the manner disclosedwith small propellent cartridges (or squibs) which are easilyattainable, replaceable, inexpensive andsafe to use. When designed foruse in association with a current limiting fuse, the arrangement isselected to be one in which the small overload current is sensed eitherby a thermal switch, relay, or similar such apparatus capable ofgenerating a small surge of energy. This surge is restricted to passthrough the firing circuit of the cartridge, and thus propel its plungerinto direct contact with a hot terminal of a current carrying line toestablish a short circuit directly to ground. Currents flowing to groundare limited in magnitude and interrupted by the current limiting fuse toprovide the needed system protection. By being able to simply replacethe plunger and propellent cartridge, a configuration is provided whichis reusable and, at the same time, small enough for adaptation into anin-line operation, or a variety of circuit protective equipmentconfigurations.

Accordingly, it is an object of the present invention to provide anautomatic grounding switch to protect against small overload currents inelectrical power distribution systems.

It is another object of the invention to provide such a switch for usein conjunction with a current limiting fuse;

It is a further object of the invention to provide such a switch whichcan be used at a minimum of expense and difficulty.

These and other objects of the instant invention will be apparent from aconsideration of the following detailed description of preferredembodiments thereof in which:

FIG. 1 is a sectional view of one embodiment of an automatic groundingswitch constructed in accordance with the principles of the presentinvention;

FIGS. 2a and 2b show top and elevational views of the acrylic mountingtube element of the automatic grounding switch of FIG. 1;

FIGS. 3a and 3b illustrates top and side views of the mounting plug andbushing assembly employable inthe grounding switch construction of theinvention;

FIGS. 4a and 4b show top and end views of the mounting tube and steelswitch body of FIG. I;

FIGS. 5a and 5b show end and side views of the copper adapter of FIG. 1;and

FIG. 6 is a schematic diagram showing one example of a sensing circuitfor use in conjunction with which the automatic grounding switch of theinvention which initiates the firing of the propellent cartridge inproviding the desired protection.

DETAILED DESCRIPTION OF THE INVENTION As shown in FIGS. 1-5, theautomatic grounding switch of the invention includes an acrylic mountingtube 10, a mounting plug 12 and a bushing 14 which screws into themounting tube 10, and a steel switch body 16 which screws into themounting plug 12. A switch 19 is provided with a downwardly extendingstud 18 which engages an opening 17a in a copper adapter 17 which islocated in the vicinity of the grounding device (and which is fixed tostud 18 by a set screw 19 through hole 17b). An acrylic insulating disc24 is shown, along with a copper plunger 26 together with a firing squib28. (A suitable squib is available as Holex catalog part No. 11968).These latter items 24, 26 and 28 are replaceable after each switchoperation wherein the copper plunger 26 is propelled by the explosivecharge contained in squib 28 to complete a short circuit includingswitch stub 18, adapter 17, plunger 26' (shown in dotted fashion),conductive switch body 16, and terminal T (held by screw 16a) to theground as a result of the sensing of a critical current level. Thedescribed assembly is arranged so as to threadedly engage the tappedopening 10a in acrylic mounting tube 10 to enable it to be easilyremoved when the mounting tube is in an inconvenient location so as tomake the disposable items listed above replaceable while the unitremains in the operators hands. It should be understood that insulatingdisc 24 maintains plunger 26 in placeand is ruptured by plunger 26 whenit is propelled by the explosive contained in squib 28. Adapter 17 isused to provide a mating surface for plunger 26 as stud 18 is normallycovered by a cable terminal 180. FIG. 6 shows a sensing circuitemployable to fire the firing squib 28 and basically comprises atemperature compensated electronic overcurrent relay having anadjustable firing point. This point will be understood to constitutethat value of primary system current which will cause the electronicovercurrentrelay to fire. The circuit has been found to be extremelywell adapted for a wide variety of applications since it is adjustableover a 50 to RMS amperes primary current range.

As shown in FIG. 6, the arrangement includes a current transformer CTinductively coupled to primary conductor PC. A series coupled neon lampNEl and a fixed resistor R1 are connected across transformer CT. Alsoshown are semiconductor diode D1, a silicon controlled rectifier Q1,ignition primer cartridge squib 28, and four capacitors C1-C4. Inparticular, the anode electrode of the diode D1 is connected to oneterminal of current transformer Cl", while'the cathode electrode of thediode is coupled in common to one terminal 280 of firing squib 28 andcapacitor C1, which is a high grade electrolytic unit. Lead 28b offiring squib 28 is connected to silicon controlled rectifier Q1, whichserially connected elements are coupled across the electrolyticcapacitor C1, with the cathode electrode of the rectifier being coupledto the lower terminal of current transformer CI. Note leads 28a and 28bof squib 28 which are also shown in FIG. 1. Five additional resistors R2R6 and a uni-junction transistor Q2 are shown, with resistor R6 being anadjustable element serially coupled between the cathode electrode of thesemiconductor diode D1 and the cathode electrode of the siliconcontrolled rectifier 01 by means of resistor R2. Resistors R3 and R4similarly couple the base electrodes of the uni-junction transistor Q2across resistor R2. The common junction between the time constantelements R5 and C2 is directly coupled to the emitter of transistor Q2.As will be understood, the sensing circuit of FIG. 6 obtains its energyfrom current transformer CT which surrounds and is inductively coupledto the primary system current conductor PC. The energy storingelectrolytic capacitor C1 is charged through the semiconductor diode D1to the peak value of the highly saturated output of this transformer.

In operation, the voltage at point A of the voltage divider formed byvariable resistor R6 and fixed resistor R2 sets the firing point for thecircuit. The circuit formed by the components including resistors, R3,R4 and R5, capacitor C2 and uni-junction transistor Q2 form a relaxationoscillator having a very stable firing voltage over a wide temperaturerange. Once the voltage at point A exceeds the starting voltage of theoscillator for a definite time interval (determined by the time constantof resistor R5 and capacitor C2), the relaxation oscillator undergoescyclic operation to feed a Turn-On" pulse to the gate electrode of thesilicon controller rectifier 01 through lead 31. In response, rectifierQ1 conducts, enabling the charge stored by capacitor C1 to be dischargedthrough low resistance firing squib 28. The energy passing through thesquib is selected to be more than sufficient to cause it to ignite, andthereby activate explosive material to propel plunger 26 towards theright (relative to FIG. 1) to rupture disc 24 and establish a shortcircuit path between the hot" terminal 18 (of switch 19) and ground.

The response time of the circuit for a particular value of fault currentis dependent on the charging time of the capacitor C1 and upon the R5,C2 time constant of the relaxation oscillator. The minimum response timefor heavy faults is primarily determined by the R5, C2 time constant,and is selected to coordinate with the minimum melt characteristic ofthe current limiting fuse employed (not shown). In this manner, it ispossible to adjust the long time interrupting characteristic of a fuseprecisely with the use of the sensing circuit. As a result, preciseoverload protection of many circuits with current limiting fuses ispossible.

The circuit is protected, furthermore, against transients and very heavyprimary system faults by the use of the neon lamp N121 and resistor R1combination, connected across the output terminal of the currenttransformer CT. Due to the relatively slow minimum response time of thecircuit (typically a few tenths of a second), normal system transientswill not generally affect the operation of the circuit.

it will thus be seen, that the automatic ground switch sensing circuiteffectively operates to provide a continuously adjustable overcurrentsensing circuit operable between 50 and 150 RMS amperes with a fixedminimum response time. At the same time, it will be seen that thesensing circuit can be made very accurate simply by using high gradecomponents such as the capacitor C1. The unit can be completelyself-contained and hermetically sealed making it both reliable andinexpensive. At the same time, the circuit is temperature compensatedand is completely adjustable with simple recalibration procedures. Oneimportant feature, again, is its immunity to normal systems transientsand overloads.

By employing such a sensing circuit, the automatic grounding switch ofthe invention can be seen to be compact, easy to maintain andinexpensive to install. Moreover, the switch is reliable, safe, andinexpensive to maintain. It will thus be seen that the automaticgrounding switch protects the fused equipment against small overloads byactivating the associated current limiting fuse when dangerousconditions exist. Such current limiting fuse technology is well known inthe art and needs no further discussion at this time.

In the foregoing, the present invention has been described in connectionwith specific illustrative embodiments. It is intended that the scope ofinvention be determined not by the specific disclosures hereincontained, however, but only by with one terminal of said currentlimiting fuserupturable means for electrically insulating said strikerand said one terminal;

first means coupled to said power distribution system conductor forstoring a voltage representative of the peak current value in saidconductor;

second means coupled to said first means for generating a firing signal;

delay means coupled to said first means for activating said second meansafter a predetermined time delay which is inversely proportional to themagnitude of current in said conductor;

ignition means coupled to said first means and responsive to said firingsignal for firing said conductive striker to rupture said rupturablemeans and make electrical contact with said current carrying conductorto effect current limiting flow through said cooperative fuse and toautomatically ground said fuse in the presence of overcurrentconditions; said first and second means and said ignition means allbeing powered exclusively by the current in the conductor beingmonitored.

2. The combination of claim 1 wherein said current carrying conductor ismaintained in a first housing, and wherein said conductive striker ismaintained in a second housing in alignment with an aperture of saidfirst housing but which is normally closed off therefrom by saidrupturable means which is comprised of an insulating disc includedwithin said second housing, and through which said striker is propelledin response to said electrical signal generation to contact said currentcarrying conductor to effect said current limiting action.

3. The combination of claim 1 wherein said first housing and saidinsulating disc are constructed of acrylic material and said secondhousing is constructed of a steel component.

4. The combination of claim 2 wherein said first means comprises anelectronic overcurrent relay circuit adjustable to provide at least thevalue of signal current needed to fire the electrically conductivestriker to contact said current carrying conductor and wherein saidrelay circuit includes a toroidal current transformer inductivelycoupled to said primary current conductor; a capacitor and a diodecoupled across said transformer for storing the peak value of thevoltage developed by said transformer.

5. The combination of claim 4 in which said second means furtherincludes a relaxation oscillator coupled to said transformer forgenerating the ignition firing signal and having an adjustableconduction point so as to vary the response time of the oscillator tomeet different anticipated overcurrent conditions.

6. The combination of claim 5 in which there is also included neon tubemeans coupled across said toroidal current transformer to protect theovercurrent relay circuit against transients and heavy primary systemcurrent faults.

7. The combination of claim 5 wherein said delay means further comprisesa resistor-capacitor circuit including an adjustable resistor foradjustably controlling the time delay.

1. In conjunction with an electrical power distribution system having aprimary current carrying conductor, the combination comprising: acurrent limiting fuse coupled to said conductor; an electricallyconductive striker operatingly cooperative with one terminal of saidcurrent limiting fuse; rupturable means for electrically insulating saidstriker and said one terminal; first means coupled to said powerdistribution system conductor for storing a voltage representative ofthe peak current value in said conductor; second means coupled to saidfirst means for generating a firing signal; delay means coupled to saidfirst means for activating said second means after a predetermined timedelay which is inversely proportional to the magnitude of current insaid conductor; ignition means coupled to said first means andresponsive to said firing signal for firing said conductive striker torupture said rupturable means and make electrical contact with saidcurrent carrying conductor to effect current limiting flow through saidcooperative fuse and to automatically ground said fuse in the presenceof overcurrent conditions; said first and second means and said ignitionmeans all being powered exclusively by the current in the conductorbeing monitored.
 2. The combination of claim 1 wherein said currentcarrying conductor is maintained in a first housing, and wherein saidconductive striker is maintained in a second housing in alignment withan aperture of said first housing but which is normally closed offtherefrom by said rupturable means which is comprised of an insulatingdisc included within said second housing, and through which said strikeris propelled in response to said electrical signal generation to contactsaid current carrying conductor to effect said current limiting action.3. The combination of claim 1 wherein said first housing and saidinsulating disc are constructed of acrylic material and said secondhousing is constructed of a steel component.
 4. The combination of claim2 wherein said first means comprises an electronic overcurrent relaycircuit adjustable to provide at least the value of signal currentneeded to fire the electrically conductive striker to contact saidcurrent carrying conductor and wherein said relay circuit includes atoroidal current transformer inductively coupled to said primary currentconductor; a capacitor and a diode coupLed across said transformer forstoring the peak value of the voltage developed by said transformer. 5.The combination of claim 4 in which said second means further includes arelaxation oscillator coupled to said transformer for generating theignition firing signal and having an adjustable conduction point so asto vary the response time of the oscillator to meet differentanticipated overcurrent conditions.
 6. The combination of claim 5 inwhich there is also included neon tube means coupled across saidtoroidal current transformer to protect the overcurrent relay circuitagainst transients and heavy primary system current faults.
 7. Thecombination of claim 5 wherein said delay means further comprises aresistor-capacitor circuit including an adjustable resistor foradjustably controlling the time delay.